1. Field of the Invention
The present invention relates in general to internal combustion engines and more particularly to a metal piston and ceramic piston pin assembly for an internal combustion engine.
2. Description of the Prior Art
A prior art piston and piston pin assembly is exemplarily shown in FIGS. 6A-6D. Referring to the figures, a piston pin 10 for the connection between a piston 11 and a connecting rod 12 is journaled in piston bosses 11a and the small end of the connecting rod 12. The bearing surfaces or bores 13 of the piston bosses 11a are formed with snap ring grooves 14 for receiving therein snap rings 15 which are brought into contact with the opposite ends of the piston pin 10 for preventing endwise movement of the piston pin 10.
In recent years, studies have been in progress as to a piston pin which is made of ceramics such as silicon nitride, sialon, silicon carbide or the like in place of metal with a view to reducing the friction loss through weight reduction, improving the fuel consumption, etc. of the engine.
By the studies, the following new problem was revealed that was not caused in the case of a metal piston pin but in the case of a ceramic piston pin. During the combustion stroke, the heat of the combusted gases is first transferred to a piston head 11b from which it is transferred to a piston circumferential wall 11c and the piston bosses 11a toward the lower end of the piston 11 sequentially. In this situation, there occurs a temperature difference between the upper part and the lower part of the piston 11, causing a difference in thermal expansion therebetween and allowing the piston 11 to deform as shown in an exaggerated manner in FIG. 6B, i.e., in such a way that the diameter of the piston 11 is largest at the upper end and reduces gradually toward the lower end. Such irregular thermal expansion, however, does not cause spotty contact between the piston 10 and the bearing bores 13 where the piston pin 10 is made of metal since the metal piston pin 10 bends in accordance with thermal expansion of the piston 11. In contrast to this, where the piston pin 10 is made of ceramics, spotty contact occurs between the piston pin 10 and the bearing bores 13 since the piston pin 10 can bend little due to its high rigidity, and simultaneously the clearance between the piston pin 10 and the bearing bores 13 of the piston bosses 11a becomes too large due to the difference of thermal expansion therebetween, making thinner the film of oil therebetween and thus causing excessive wear of the bearing bores 13 at the particular portions 13a thereof located closer to the piston head 11b as shown in FIGS. 6C and 6D.